To make a funnel, we take a circular piece of metal, cut out a sector,
and connect the two radial edges together to make an open cone. What
should the angle of the sector be to maximize the volume of the cone?

I am trying to calculate the midpoint between cases of legionella and
their nearest neighbor. How can I calculate the distance between two
points below which they can be treated as if they were in a plane
rather than on a sphere?